Mrs May will tell Mrs Merkel that she wants to send a “clear message about the importance” of Britain’s relationship with Germany “not just now but also when we have left the European Union”.

The target to reduce net migration to the “tens of thousands” dogged David Cameron’s time in office as the number of foreigners moving to the UK spiralled during his six years as Prime Minister.

Ahead of last year’s general election, the “target” was downgraded to an “ambition”, in a move that was criticised by Tory backbenchers.

The issue of net migration was the centrepiece of the campaign to leave the EU. Net migration soared to 335,000 last year.

Speaking during her first appearance since being promoted to Home Secretary, Ms Rudd said: “What the Prime Minister has said is that we must bring migration down to sustainable levels so that's what is going to be my aim at the moment."

Pressed on whether the net migration target had changed, Ms Rudd said: "I'm going to stick to my comment which is about bringing it down to sustainable levels. That has to be the most important thing for the country."

Later at a press conference alongside John Kerry, the US secretary of state, Mr Johnson said that Ms Rudd was “entirely right to be careful about committing to numbers because one doesn't want to be in a position where you are disappointing people again”.

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He said: "What is certainly possible, post leaving the EU - and once we end our obligations under uncontrolled free movement - it will be possible to have a system of control.

"This is what we were talking about in the referendum campaign. You can't do that immediately, clearly, because we are still in the EU and subject to uncontrolled free movement."

Mrs May’s spokeswoman said: “In the Prime Minister's view sustainable levels does mean the tens of thousands but we should also recognize the work that will be needed to do that and to get us down to sustainable levels.”

Lord Green of Deddington, chairman of MigrationWatch, which campaigns for tougher border controls, said: “The history of recent years proves that a target is invaluable to achieve the policy focus which the public most certainly most want to see.

“To abandon it, if that is what is suggested, would be a very serious mistake.”

Peter Bone, the MP for Wellingborough, said: “Obviously it’s a manifesto commitment so we are bound to implement the government’s policy on that.

“The country’s voted to come out of the EU and the biggest single issue in that was ending freedom of movement to bring the numbers down for those coming here.

“I’m sure the new Home Secretary will want to support government policy as I do.”

He added: “It is a non-starter. It would fly in the face of what the referendum was about. It would be absurd if we broke our pledge on that. It wouldn’t happen and can’t happen."

Another Conservative MP said: “If we’re abandoning manifesto pledges made only a year ago it’s going to heighten calls for general election to seek a new mandate.”